An implantable eye pressure monitor for glaucoma patients is believed to contain the first complete millimeter-scale computing system.
Nearly invisible millimeter-scale systems could enable ubiquitous computing, says professor David Blaauw of the University of Michigan.
"When you get smaller than hand-held devices, you turn to these monitoring devices. The next big challenge is to achieve millimeter-scale systems, which have a host of new applications for monitoring our bodies, our environment and our buildings," he says.
"Because they're so small, you could manufacture hundreds of thousands on one wafer. There could be tens to hundreds of them per person, and it's this per capita increase that fuels the semiconductor industry's growth."
The team's pressure monitor is designed to be implanted in the eye to continuously track the progress of glaucoma, a potentially blinding disease.
In a package just over one cubic millimeter in size, the system fits in an ultra low-power microprocessor, a pressure sensor, memory, a thin-film battery, a solar cell and a wireless radio with an antenna that can transmit data to an external reader device that would be held near the eye.
The researchers' Phoenix processor uses a unique power gating architecture and an extreme sleep mode to achieve ultra-low power consumption. To keep the battery charged, it requires exposure to 10 hours of indoor light each day or 1.5 hours of sunlight. It can store up to a week's worth of information.
Also new from the University of Michigan is a similarly tiny radio, with an on-chip antenna that doesn't need a bulky external crystal. Instead, the new antenna keeps time on its own and serves as its own reference.
"Antennas have a natural resonant frequency for electrical signals that is defined by their geometry, much like a pure audio tone on a tuning fork," says assistant professor david Wentzloff.
"By designing a circuit to monitor the signal on the antenna and measure how close it is to the antenna's natural resonance, we can lock the transmitted signal to the antenna's resonant frequency."
The researchers are now working on lowering the radio's power consumption so that it's compatible with millimeter-scale batteries.